1. Field of the Invention
The present invention relates to a carrier frequency communication transmission system having a premodulation band lying above the transmission position of the primary group band of 60-108 kHz, and more particularly to such a system in which mechanical filters are utilized as channel filters and also a common group filter in order to filter out the primary group band.
2. Description of the Prior Art
In order to transpose 12 voice channels into the base primary group band position of a TF system, the premodulation technique has been proven advantageous. The frequency spectrum of an input signal in the premodulation technique is displaced into the premodulation position in a first transposition step. A channel filter connected to the output of the modulator in this step limits the transmitted frequency band and suppresses undesirable modulation products. In a second conversion step, the frequency spectrum is displaced into the frequency position of the primary group band, i.e. 60-108 kHz. The undesirable modulation parts of the second conversion step are suppressed in the group filter.
It is already known in the art to establish the premodulation band in the frequency range of 48-52 kHz and to arrange the channel carrier frequencies above the primary group band. In this connection, for example, see the German Allowed Application No. 1,274,663. In this system, a common group filter is utilized for filtering out the primary group band arranged in inverted frequency, the group filter being designed as a band pass filter. Mechanical filters, designed as longitudinally coupled bending resonators, are utilized as channel filters for this purpose.
Such a system has the advantage that no multiples of the input signal or undesired summation of differential products of the input signal with the carrier frequencies fall into the higher transmission positions during additional conversions. The bending resonators in this frequency range are both small in dimension and can be produced economically.
The article "Mechanical Filters for the Carrier Frequency Techique", by M. Boerner, published in the periodical "NTF", 19, 1960, pp. 34-37, describes a premodulation system having mechanical filters in which the premodulation band lies at 200 kHz, i.e. above the primary group band, whereby torsion resonators having longitudinal coupling are utilized as mechanical filters. On the basis of the essentially greater dimensions of mechanical filters with torsion resonators in contrast to mechanical filters with bending resonators, their utilization in a frequency band below the primary group band is not possible if one is bound to prescribed dimensions, particularly provided by the standardized printed circuit plates. This known premodulation system also utilizes a common group filter in order to filter out primary group.
Furthermore, a mechanical filter having bending resonators is known from the German Letters Pat. No. 1,541,975, these resonators being coupled together by coupling wires attached to the center, or close to the center whose support is low in feedback via respectively two holding wires which are attached to the bending node and which, for example, are driven by piezo electric transducers.